Driver's side head you say? There is a freeze plug as you noted back there that if it leaks will spill coolant down the back. However, there is also a freeze plug on the back of the block to the left of the oil plug that can spill coolant in that area as well. The only way to service that one is to remove the transmission. If you mean from under the exhaust manifold there is one freeze plug located near the engine mount bracket on the driver's side. Otherwise the actual coolant plumbing is all routed down the passenger side with exception for the the lower rad hose which is driver's side but in the front. One other thought I just had was the rear of the lower intake valley. If that has an external leak that would appear at the back of the block and would probably drip coolant down and off the front of the transmission bellhousing.
The correct thing to do with a leaking freeze plug if that is the issue is to remove and replace. That however can involve copious amounts of time and effort providing you have a place to wrench on your truck. If you isolate it to a freeze plug and the leak isn't horrible you could try patching it with JB Weld or something like that for a temporary fix. I must emphasize JB welding a freeze plug is generally a very bad idea. Most of the time when they leak it's because they are rusting out and paper thin in places. JB Weld will not stop a rusty plug from rusting out worse. Also, if one is rusting out generally the others will be soon to follow.
If the freeze plugs look ok and you don't have a leak from the valley in the back then you should be thinking about a cracked head or or leaking head gasket or cracked block(very rare).
I know this sounds like a lot to check but break it down by category and start looking. If you have any further questions I'll do the best I can to help. I went through this exact process on my '94 B3000.
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